Controlled Photoluminescence Properties Of CdSe Quantum Dots And Fabrication Of Highly Efficient Colloidal Quantum Dot Light-Emitting Diodes

Semiconductor quantum dots (QDs) with size-tunable emission properties have been widely applied in light-emitting diodes (LEDs), photovoltaic cells, and biological labels. The photoluminescence (PL) quantum yield (QY) of QDs reaches 85%, after two decades’development of synthesis technology for QDs. QD based quantum dot light emitting diodes (QD-LEDs) have potential to be the next generation solid state lighting source for display and illumination due to its size-tunable emission wavelength, narrow emission full width at half maximum (FWHM), high PL QY and thermal stability. Recently, the performances of QD-LEDs have a great improvement, for example the external quantum efficiency (EQE) of QD-LEDs is increased from 0.001% to 7%, which approaches to that of organic light emitting diodes (OLEDs). However, the photo-physics processes of core/shell QDs and the electroluminescence (EL) mechanism of QD-LEDs are still not well understood. Therefore, the study for the effects of shell structures of core/shell QDs on the PL and EL mechanism is very important to optimize the performances of QD-LEDs. We study the temperature dependent PL properties of core/shell QDs, energy transfer/ charge separation of QD/ETM or HTM blend films and EL properties of the core/shell QDs. The original works are organized as follows: 1. The photoluminescence spectra of CdSe-core CdS/CdZnS/ZnS-multishellquantum dots were studied to understand the radiative and nonradiative relaxationprocesses in the temperature range from 80 to 360 K. The mechanism oftemperature-dependent nonradiative relaxation processes in the CdSe QDs withchanging the shell structures was found to evolve from thermal activation of carriertrapping by surface defects/traps in CdSe core QDs to the multiplelongitudinal-optical (LO) phonon-assisted thermal escape of carriers in the core/shellQDs. An increase in PL intensity with increasing temperature was clearly observed inthe core/shell QDs with a thick CdS monoshell and a CdS/ZnCdS/ZnS multishell. ThePL enhancement was considered to come from delocalization of charge carrierslocalized at the CdSe/CdS interface with the potential depth of 30 meV. Theexperimental results indicated that the improvement of PL quantum efficiency inCdSe-core CdS/CdZnS/ZnS-multishell QDs could be understood in terms of thereduction of nonradiative recombination centers at the interfaces and on the surface ofthe multishell, as well as the confinement of electrons and holes into the QDs by anouter ZnS shell.2. The steady-state and time-resolved photoluminescence spectroscopy wasused to study the energy transfer between CdSe core/shell quantum dots and1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene (TPBI) in inorganic/organic blendfilms The shortening in PL lifetime of TPBI molecules and the resulting lengtheningin PL lifetime of the QDs demonstrated an efficient energy transfer process fromdonor to acceptor. The slowest PL decays of CdSe core/shell QDs observed in theblend films with low QD concentration were considered to result from the maximumenergy transfer process from the surrounding TPBI molecules of a QD to itself. ThePL decay curves of the core/shell QDs with a CdS, ZnS, and CdS/ZnCdS/ZnS shellswere simulated to obtain the excited state lifetimes of the surrounding TPBImolecules for understanding the effect of the shells on the energy transfer process. Itwas surprisingly found that the obtained energy transfer rate to a QD with a thickCdS/ZnCdS/ZnS multishell from the surrounding TPBI molecules with the maximumcontribution of the energy transfer was almost the same as that to a QD with a thinZnS monoshell and smaller than that to a QD with a CdS monoshell. Theexperimental results indicated the energy level alignment and the structure of shells inCdSe core/shell QDs determined the energy transfer efficiency from TPBI moleculesto the core/shell QDs. 3. The steady-state and time-resolved photoluminescence spectroscopy was used to study the energy transfer/ charge separation processes in quantum dots and hole transporting materials (HTMs), 4,4’,4’’-Tris(carbazol-9-yl)-triphenylamine (TCTA) or N,N-diphenyl-N,N’- bis(3-methylphenyl)-(1,1’- biphenyl)-4,4’-diamin (TPD) blend films. Due to the hole capture ability of HTMs, the PL lifetime of CdS shell coated QDs embedding in TCTA or TPD matrix was significantly shortened. However, the PL lifetime of CdSe/CdS/ZnCdS/ZnS core/multi-shell QDs was not changed in HTMs matrix, indicating that the multi-shell suppressed the charge separation at QD/HTM interface. Fitting the PL decay profile of core/multi-shell QDs in TCTA with low QD concentration, the energy transfer efficiency from surrounding TCTA molecules to the central QD was obtained to 35.6%. The experimental results indicated the energy levels of HTMs and shell structure of QDs control the energy transfer/ charge separation in QD/HTMs blend films.4. The colloidal CdSe/CdS, CdSe/ZnS, and CdSe/CdS/CdZnS/ZnS core/shell quantum dots were fabricated in multilayer light-emitting diodes by spin coating a near monolayer of the core/shell QDs on cross-linkable hole transporting layers. It is found that CdSe/CdS QD-LEDs exhibit a faster decrease in EL quantum efficiency (2% at a brightness of 100 cd/m2) with increasing current density and lower maximum brightness than those of CdSe/ZnS QD-LEDs. A more significant redshift and spectral broadening of the EL observed in CdSe core/ shell QDs with a CdS or CdS/CdZnS/ZnS shell than with a ZnS shell indicate that the electron wave function can penetrate into the shell under electric field. The difference in device performance and EL spectra results from conduction band offsets between the CdSe cores and CdS or ZnS shells, suggesting the existence of the exciton ionization in the QD-LEDs.